The present invention relates to free-space optical communication terminals and, more specifically, to an apparatus and method for increasing capture area in optical terminals.
High-bandwidth, free-space optical (FSO) communication terminals generally fall into two categories: single- or multi-mode fiber coupled. This refers to the type of fiber used to couple the received optical signal from free space into an optical fiber for interfacing to high-bandwidth, back-end systems.
Single-mode terminals are generally more complex as they have to place the incoming signal on a much smaller target on the order of 8-10 μm as opposed to multi-mode terminals where the target can be 50 or 63 μm. The tradeoff is that the back-end system, the transceivers and optical auto gain controllers (OAGCs), have much better performance for single-mode vs. multi-mode systems.
Most importantly, for a communication system, single-mode systems can leverage lower noise optical amplifiers to achieve lower receiver sensitivities. This yields much more link margin as compared to multi-mode receivers. For example, a 10 Gbps single-mode system operating at a wavelength of 1550 nm can achieve a receiver sensitivity of −40 dBm as compared to −26 dBm for a multi-mode system. This additional link margin is highly desired to achieve high-throughput FSO communication links.
The cost of using a single-mode system is complexity in the optical terminal. Generally, single-mode terminals require a high-performance adaptive optics system to correct wave-front distortions of the optical signal in order to focus it on the receive fiber.
What is needed then is an increase in the area or acceptance angle of the receive fiber in order to lessen the pointing requirements of the terminal and thereby reap the other benefits of a single-mode system.